I installed Postfix on an Ubuntu 14.04 server running Nginx. While I was installing it, I put the system email as 'info@mydomain.com' not realising that it would now think that this was a local mailbox. This email address is a gmail account and so the server needs to look at the MX records.
I'm guessing what is happening is that it sees that its a local address and just sends it locally. I sent a test with:
echo "This is the body of the email" | mail -s "This is the subject line" info@mydomain.com
And sure enough in /var/mail/root there was a message:
From MAILER-DAEMON Wed Sep 30 19:05:59 2015
Return-Path: <>
X-Original-To: root@mydomain.com
Delivered-To: root@mydomain.com
Received: by mydomain.com (Postfix)
id 5D29F1249E9; Wed, 30 Sep 2015 19:05:59 -0400 (EDT)
Date: Wed, 30 Sep 2015 19:05:59 -0400 (EDT)
From: MAILER-DAEMON@mydomain.com (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: root@mydomain.com
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="52BC21249E1.1443654359/mydomain.com"
Message-Id: <20150930230559.5D29F1249E9@mydomain.com>
This is a MIME-encapsulated message.
--52BC21249E1.1443654359/mydomain.com
Content-Description: Notification
Content-Type: text/plain; charset=us-ascii
This is the mail system at host mydomain.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<info@mydomain.com>: unknown user: "info"
--52BC21249E1.1443654359/mydomain.com
Content-Description: Delivery report
Content-Type: message/delivery-status
Reporting-MTA: dns; mydomain.com
X-Postfix-Queue-ID: 52BC21249E1
X-Postfix-Sender: rfc822; root@mydomain.com
Arrival-Date: Wed, 30 Sep 2015 19:05:59 -0400 (EDT)
Final-Recipient: rfc822; info@mydomain.com
Action: failed
Status: 5.1.1
Diagnostic-Code: X-Postfix; unknown user: "info"
--52BC21249E1.1443654359/mydomain.com
Content-Description: Undelivered Message
Content-Type: message/rfc822
Return-Path: <root@mydomain.com>
Received: by mydomain.com (Postfix, from userid 0)
id 52BC21249E1; Wed, 30 Sep 2015 19:05:59 -0400 (EDT)
Subject: This is the subject line
To: <info@mydomain.com>
X-Mailer: mail (GNU Mailutils 2.99.98)
Message-Id: <20150930230559.52BC21249E1@mydomain.com>
Date: Wed, 30 Sep 2015 19:05:59 -0400 (EDT)
From: root@mydomain.com (root)
This is the body of the email
--52BC21249E1.1443654359/mydomain.com--
I'm not sure how I can get the server to send the emails to the right place? Should I change the system email to something random?
I'm guessing that you've probably got the machine installed with mydomain.com as the hostname and so Postfix is configured to think it's answering email for mydomain.com. You probably want to edit your /etc/postfix/main.cf and remove your domain from the mydestination line there. (And then restart Postfix.)